The invention lies in the telecommunications field. More specifically, the invention relates to a method and a radio station for data transmission via a radio interface in a radio communications system. The invention is particularly suited for a mobile radio network.
In radio communications systems, information (for example voice, video information, or other data) is transmitted using electromagnetic waves. The electromagnetic waves are thereby transmitted at carrier frequencies which are in the frequency band intended for the respective system. For GSM (Global System for Mobile Communications), the carrier frequencies lie in the 900 MHz range. For future radio communications systems, for example the UMTS (Universal Mobile Telecommunications System) or other 3rd generation systems, frequencies are provided in the frequency band around 2000 MHz.
The transmitted electromagnetic waves are attenuated by losses due to reflection, refraction, and transmission losses due to the curvature of the earth. This results in a reduction in the reception power which is available at the receiving radio station. This attenuation is position-dependent. In the case of moving radio stations it is time-dependent as well.
Between a transmitting and a receiving radio station, there is a radio interface via which data transmission takes place using electromagnetic waves. In an earlier disclosure (copending application No. 09/495,794; German published application DE 197 33 336) we described a radio communications system which uses CDMA subscriber separation (CDMA=Code Division Multiple Access), with the radio interface also having time-division multiplex subscriber separation (TDMA=Time Division Multiple Access). A JD (Joint Detection) method is used at the receiving end in order to use knowledge of the spread codes of a plurality of subscribers to improve the detection of the transmitted data. It is thereby known that a connection via the radio interface can be assigned at least two data channels, in which case each data channel can be distinguished by means of an individual spread code. Jung P. et al., xe2x80x9cA joint detection CDMA mobile radio system concept developed within COST 231xe2x80x9d, Proceedings of the vehicular technology conference, Chicago, Jul. 25-28, 1995, Vol. 1, No. Conf. 45, pages 469-73, discloses a radio system with a combined TDMA/CDMA radio interface, in which a plurality of signals, which can be separated by means of CDMA codes, are transmitted in one timeslot.
It is known from the GSM mobile radio network for transmitted data to be transmitted as radio blocks (bursts), with midambles with known symbols being transmitted within a burst. These midambles may be used as training sequences in order to tune the radio station at the receiving end. The receiving radio station uses the midambles to estimate the channel impulse responses for various transmission channels.
Bursts having midambles or having one or more data sections which can be distinguished by means of an individual fine structure are added and are digitally and/or analog-filtered before transmission, at the transmission end. Inhomogeneous deformation of the signal sections has been observed, after filtering, during simulations on a demonstrator. At the receiving end, this leads to fluctuations in the reception quality of the data transmission.
It is accordingly an object of the invention to provide a method and a radio station for data transmission in a radio communications system, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which further improve data transmission.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of transmitting data via a radio interface in a radio communications system, which comprises transmitting at least one subscriber signal carrying individual fine structures via a radio interface, the signal containing data symbols in data sections and a midamble with known symbols, and the fine structures of the data sections and of the midamble differ;
summing at a transmission end to produce a sum signal containing signal sections to which different fine structures are applied;
filtering the sum signal in at least one transmission filter and transmitting via a transmission channel to a receiving radio station at a receiving end; and
controlling the summing with a control device such that different attenuation of the signal sections is compensated for by the filtering.
In other words, to transmit data via a radio interface in a radio communications system, at least one subscriber signal is transmitted, which has an individual fine structure applied to it and, in addition to data symbols in data sections, contains a midamble with known symbols, wherein the fine structures of the data sections and of the midamble differ. At the transmission end, a sum signal is produced which contains signal sections to which different fine structures are applied. The sum signal is filtered in at least one transmission filter and is transmitted via a transmission channel to a receiving radio station. A control device controls the addition so that different attenuation of the signal sections is compensated for by the filtering.
The invention is based on the knowledge that the deformation is caused by the filtering, with the attenuation levels of the individual signal sections differing considerably (up to 2 dB in an AWGN channel). The fine structures do not all have the same characteristics with regard to the filtering or with regard to the transmission characteristics in specific transmission channels. The fluctuations in the reception quality can be reduced by the measures according to the invention for controlled addition of the signal sections.
In accordance with an added feature of the invention, only one subscriber signal is transmitted from a mobile station to a base station, and the summing step is carried out such that a mean power level of the sum signal of the data sections and of the midamble is approximately the same after filtering. This advantageous development of the invention relates to a mobile station. If only one subscriber signal is transmitted to a base station, then the addition is carried out in such a manner that the mean power level of the sum signal of the data sections and of the midamble is approximately the same after filtering. The signal sections are thus formed by the midamble and the data section, with the fine structures of the two parts differing when spreading is carried out using CDMA codes and thus leading to different attenuation levels. This is undesirable, since transmission power levels which have been set for both signal sections are intended to guarantee adequate reception. The method according to the invention reduces the deformation of the sum signal in the transmitting mobile station itself.
For mobile stations which have to manage with amplifier arrangements that can be produced at low cost, a constant envelope is particularly important, since the dynamic requirements for the amplifier arrangement can be reduced. According to one development of the invention, the sum signal is transmitted as a burst in one timeslot, wherein the mean power level is set in such a manner that the burst has a constant envelope. Different attenuation levels between the data section and midamble are regulated out. The setting of the power ratio between the midamble and the data section is advantageously based on a constant dynamic range. This means that, within a dynamic range which is predetermined for the entire burst, the transmission power of the midamble or of the data section is set in a suitable manner for improved transmission.
At least two subscriber signals are transmitted via the radio interface for mobile stations using a variable data rate or using different transmission services, and for base stations. In accordance with another feature of the invention, the addition of the subscriber signals is carried out in such a manner that different attenuation levels, which can be associated with individual fine structures, in the individual subscriber signals are compensated for. In this case, the signal sections are formed by the different data sections in the subscriber signals, which are likewise deformed differently by the filtering. Appropriate power regulation during the addition results in the power ratios of the subscriber signals at the filter output corresponding to the desired relationships.
In accordance with a further feature of the invention, only one midamble is used for a plurality of connections. Only a portion of the midamble is thus evaluated for each connection at the receiving end. In this case, the transmission power level for the midamble is set for all the connections, rather than on a connection-specific basis. This measure simplifies the implementation of the addition and burst formation.
In accordance with again an added feature of the invention, in order to compensate for the attenuation levels, channel values are stored, which are associated with individual filtering-dependent attenuation levels of spread codes which form the individual fine structure. These channel values are taken into account for controlling the power level of the sum signal. The channel values describe, for example, the relative attenuation of various spread codes in comparison to a midamble. If the filter conditions are constant over time, then, in consequence, the data sections can each have a fixed channel value applied to them for the addition.
The channel values for a group of spread codes that is used are measured and stored in a radio station; alternatively the channel values of the spread codes are transmitted together with the selection of the spread codes, and are then stored in the radio station. The first solution provides advantages if the number of spread codes used is relatively small, while the second solution allows greater flexibility.
The channel values advantageously also include attenuation levels for the transmission channel and/or of reception filters in the receiving radio station. The transmission channel and the reception filters also lead to different distortion to signal sections. Transmission-end compensation for these attenuation levels that are dependent on the fine structure as well additionally improves the transmission quality. One particularly advantageous detection algorithm for the receiving end is a so-called JD detection algorithm. For this algorithm, it is advantageous for the channel values also to take account of the influences of different processing gains. These improvements to the attenuation compensation at the same time make the power regulation between the transmitting and receiving radio stations less sensitive and less susceptible to interference.
In accordance with again an additional feature of the invention, the channel values are calculated in advance for specific radio cell environments. Channel conditions can be described by typical channel models, such as AWGN (additive white gaussian noise), RA (rural area), TU (typical urban), BU (bad urban). Radio cells can be associated with such channel models. The attenuation of individual spread codes can be calculated in advance for these channel models and can be assigned to the radio cells as channel values for further use. If the channel values are calculated using an AWGN channel model, then power regulation results in the spread codes having the same response in indoor and outdoor areas. Wave propagation takes place directly in these radio cell environments. If the optimization is carried out for a TU channel or BU channel, then this provides attenuation compensation for radio cell environments that are subject to severe multipath propagation.
In accordance with again another feature of the invention, the attenuation levels are also compensated for by a group of fine structures being selected for one timeslot for a radio interface with a TDMA component, which group results in approximately the same attenuation ratios in all the subscriber signals. Additional computation complexity for addition and burst formation can be reduced, or even entirely avoided.
With the above and other objects in view there is also provided, in accordance with the invention, a radio station (a base station or a mobile station) for transmitting data via a radio interface in a radio communications system, wherein at least one subscriber signal is transmitted via the radio interface, the subscriber signal having an individual fine structure applied to it and containing data symbols in data sections and a midamble with known symbols, wherein the fine structures of the data sections and of the midamble differ. There are provided:
an addition element for summing signal sections to which different fine structures are applied, to form a sum signal;
at least one transmission filter connected to the addition element for filtering the sum signal; and
a control device connected to and controlling the addition element such that different attenuation of the signal sections is compensated for by the filtering of the sum signal.
In accordance with a concomitant feature of the invention, wherein the radio station is a base station of a mobile radio network, including a transmitting device for transmitting at least two subscriber signals via the radio interface, and wherein the addition element is designed such that attenuation levels, which can be associated with the individual fine structures, of individual subscriber signals are compensated for.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in method and radio station for data transmission, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.